Aperiodic low-pass filter



April 23, 1968 F. N. HANSEN 3,380,004

APERIODIC LOW-PASS FILTER Filed Jan. 29, 1962 2 Sheets-Sheet 1 (I, d m80 6 J O 0 g z J 5- 4 I! I l l l l I FREQUENCY IN MEGACYLES PER SECONDFIG. 2

INVENTOR.

FRITHJOF N. HANSEN BY 1M 5W ms ATTORNEY April 23, 1968 F. N. HANSEN3,380,004

APERIODIC Low-PAss FILTER Filed Jan. 29, 1962 2 Sheets-Sheet 2 FIG.4

INVENTOR.

FRITHJOF HANSEN BY M. HIS ATTORNE United States Patent 3,380,004APERIODIC LOW-PASS FILTER Frithjof N. Hansen, Beverly Farms, Mass.,assignor to The McMillan Corporation of North Carolina, Raleigh, N.C., acorporation of North Carolina Contiuuation-in-part of application Ser.No. 787,885, Jan. 20, 1959. This application Jan. 29, 1962, Ser. No.177,131

13 Claims. (Cl. 333-79) This invention relates to filters for permittingthe transmission of electrical power of desired low frequencies whilesupporting the transmission of power of undesired high frequencies.

More particularly, the invention relates to an aperiodic low-pass filterwith uniformly distributed inductance and capacitance.

This application is a continuation-in-part of my copending applicationSer. No. 787,885 filed on Jan. 20, 195-9.

Briefly, the invention consists of a power conductor, a conductiveshield surrounding the power conductor and insulated therefrom, and acore of finely divided magnetic material mixed with an epoxy resinbinder interposed between the conductor and the shield.

The invention will be more fully understood upon reading the followingspecification, reference being had to the accompanying drawing forming apart thereof.

Referring to the drawing:

FIGURE 1 is a view in longitudinal section of a lowpass filter embodyingthe invention.

FIGURE 2 is a graph showing the frequency-attenuation performance of thefilter of FIG. 1.

FIGURE 3 is similar to FIG. '1 showing a filter equipped with insulatingterminals and mounted on a metal wall.

FIGURE 4 shows a filter similar to the filter of FIG. 1 equipped withshunt capacitors which also serve as its terminals.

Referring to FIG. 1, a power conductor 11 is shown as having a helicalconfiguration intermediate its ends. The power conductor 11 is laterallysurrounded by a length of conductive tubing 12 which also serves as acase for the filter. Both the conductor 11 and the tubing 12 should beof material having a relatively high conductivity, such as copper. Justinside conductive tubing 12 in FIG. 1, is shown an insulating sleeve 13which may be a distinct component or may simply be an insulating coatingapplied to the interior surface of conductive tubing 12. In the filtershown in FIG. 1, I employed mica-coated Fiberglas in the construction ofinsulating sleeve 13. The interior of sleeve 13 including the spacebetween the turns of the helix of conductor 11 and between conductor 11and insulating sleeve 13 is filled with a magnetic material 14 whichserves as a core for the helix and also as a lossy dielectric materialfor the capacitor formed by filamentary conductor 11 and conductivetubing 12. To some extent, magnetic material 14 itself is suificientlyconductive so that it serves as a plate of a capacitor the other plateof which is conductive tubing 12.

Magnetic material 14 comprises iron which has been powdered to such anextent that the particles are rather fine. Of the iron particles in thefilter core shown in FIG. 1, 23 percent are smaller than 250 mesh, whilethe remainder are larger than 25 0 mesh. The power conductor 11 may besupported by a jig within conductive tubing 12 and insulating sleeve 13while magnetic material 14 is introduced to fill all the void spaces. Ifdesired, the magnetic material may be introduced under vacuum in order"ice that the amount of trapped air may be reduced to a minimum.

A point of interest in the construction of my filter is that filamentaryconductor 11 should be in electrical contact with magnetic material 14.While it is possible, by means of an insulating coating on filamentaryconductor 11, to permit magnetic material 14 to be in conductive contactwith conductive tubing 12, it is preferable from an electricalstandpoint to use insulating sleeve 13 between conductive tubing 12 andmagnetic material 14 while permitting magnetic material 14 to be inintimate electrical contact with filamentary conductor 11. In otherwords, since it is necessary only that either filamentary conductor 11or conductive tubing 12 to be insulated from magnetic material 14,electrical considerations render it desirable for the insulation to beapplied to conductive tubing 12 rather than to filamentary conductor 11.In this way, skin effect currents of high frequency flow in the closestpossible proximity to the magnetic material 14.

A further point of interest in regard to magnetic material 14 is thefact that its conductivity must be finite rather than zero, and yet itsconductivity must be much less than that of filamentary conductor 11. Itis clear that, while it must be possible for electrical loss to takeplace in magnetic material 14, the greater part of the power currentshould still flow through conductor 11 rather than through magneticmaterial 14. I have found that a good operational relationship betweenthe conductivity of filamentary conductor 11 and that of magneticmaterial 14 in bulk is the ratio to 1. Of course, I do not intend tolimit myself to conductivities having that ratio.

Although power conductor 11 has been shown and described as beinghelical in configuration within conductive tubing 12, it will beapparent that the pitch of the helix could become so large thatfilamentary conductor 11 would approach the configuration of a straightwire. [In fact, a filter may be constructed in which power conductor 11is a straight wire throughout its entire length. However, it is clearthat, by employing the helical configuration for conductor 11, I am ableto increase its distributed series inductance per unit length.

Still another point of interest depends upon the fact that all of theconvolutions of the helix of filamentary conductor 11 are equidistantfrom the internal surface of conductive tubing 12 and hence, eachincrement of a given length of filamentary conductor 11 has capacitanceto conductive tubing 12 which is comparable with the capacitance ofevery other increment of the same length of filamentary conductor -11.While it will be apparent that some increments of filamentary conductor11 are closer than others to the ends of conductive tubing 12 andtherefore have capacitances to the grounded tubing 12 which differ tosome degree. Nevertheless, the capacitance along the helix ischaracterized by a smooth, uniform distribution. Consequently, and byreason of the lossy nature of magnetic material 14, the filter isaperiodic and free from any sharp peaks in the frequency responsecharacteristic. Such aperiodic performance is in sharp contrast to thatof the prior art filters which display peaks and valleys in theirfrequency response characteristics. This aperiodic performance islargely attributable to the use of the core 14 of magnetic material ofmoderate conductivity.

In operation, the conductive sleeve 12 tends to simulate the outerconductor of a coaxial transmission line at higher frequencies. Thiswave transmission line action, however, is impeded by the moderatelyconductive magnetic material 14 which operates as a lossy obstructingplug. In order to enhance the obstructing effect of the lossy magneticmaterial 14, the thickness of the insulating sleeve 13 should be kept toa minimum.

It is clear that a number of variations can be made in the filter ofFIG. 1 without departing from the essence of this invention. Forinstance, instead of a Fiberglas insulating sleeve 13, one might employa film of lacquer or high dielectric plastic material sprayed or brushedupon the internal surface of conductive tubing 12. Alternatively, itwould be possible to apply such an insulating coating to the outersurface of a subassembly consisting of the filamentary power conductor11 and lossy magnetic material 14, and then apply conductive tubing 12by spraying or painting highly electrically conductive material over theinsulative coating previously applied to the moderately conductivemagnetic material 14. Some method would have to be employed formaintaining the mechanical stability of the magnetic material until theinsulated conductive tubing is in place around the magnetic material.One way to obtain such a construction is to form an inner core ofmagnetic material, such as powdered iron in cylindrical form, and sinterit so that it will hold its shape while the turns of the power conductor11 are applied around it. If desired, this inner core may be formed andsintered in a container having helical depressions formed on its innersurface, whereby the inner core is produced with a threaded surfacesuitable for receiving the convolutions of the filamentary powerconductor 11. When these turns have been applied to the inner core, thesubassembly may be supported within the insulated conductive tubingwhile the remainder of the magnetic material 14 is put in place, therebycompleting the core .and filling in the spaces between turns of thehelix and the insulated inner surface of the conductive tubing.

With regard to the specific physical dimensions of the filter of FIG. 1,conductive tubing 12 is a 5 inch length of copper tubing having anoutside diameter of inch. Power conductor 11 is formed of No. 14 copperwire wound into a helix providing 24 turns within the 5 inch length ofconductive tubing 12. Magnetic material 14 comprises ordinary powderediron designated No. 5-P38-5, which is inexpensive and easily obtained onthe market. These dimensions give the filter a uniformly distributedinductance of approximately 4 microhenries and a uniformly distributedcapacitance of approximately 800 micromicrofarads. It will be understoodthat these parameters, while appropriate for the performancecharacteristic of FIG. 2, are by no means to be taken in a limit ingsense. The effective inductance of the filter may be increased byincreasing the length of the power conductor 11, as by increasing thenumber of turns in its helical configuration. The spacing betweenadjacent turns must, however, be kept sufiiciently large to preventeificient interturn coupling. The capacitance to ground is determined bythe thickness of the insulation between the lossy magnetic material 14and the conductive tubing 12 which is provided in FIG. 1 by insulatingsleeve 13 and also, to a certain extent, by the proximity of the turnsof the helix to the conductive tubing 12.

While the filters described in this specification are straight inexternal over-all configuration, it is clear that filters according tomy invention need not always be straight and in fact might necessarilybe other than straight under some circumstances. A possible occasion formaking a filter other than straight would occur if a very long filterwere required and the space to accommodate it were limited. In thatevent, it might be advantageous to form the entire filter into a coil.The use of a iflexible power conductor and flexible conductive tubingwould facilitate the production of filters having other than rectilinearconfigurations.

Turning now to FIG. 2 of the drawing, there is shown a graph of thefrequency-attenuation performance characteristics of the filter shown inFIG. 1. The plot shows, as a function of frequency, the attenuation ofcurrents passing through the filter, where the input power is connectedto conductor -11 with conductive tubing 12 grounded. It will be notedthat the attenuation begins, with increasing frequency, at a rather lowvalue and rises sharply with further increasing frequency to a valuebeyond which it can no longer effectively be measured. It is possible,however, to state with assurance that the attenuation for these higherfrequencies is in excess of 120 decibels and that there are no points ofresonance where the attenuation reverts below that level in the upperfrequency ranges. This is an important feature in view of therequirements which have been established for filters for use inconnection with shielded rooms, for example, and which require that theyattenuate all energy components having frequencies of 200 megacycles persecond or higher by at least 120 decibels. While the performance of thefilter at frequencies of 200 megacycles or higher is completelysatisfactory, special design considerations become important in theoptimization of performance at frequencies of megacycles per second orless. In order to extend the band of acceptable attenuation tofrequencies which are very low indeed, shunt capacitors may be connectedbe tween the power conductor 11 and the conductive tubing 12 at eitheror both of its ends. Such a construction is illustrated in FIG. 4 anddescribed in greater detail below. It is important to realize, however,that even without the use of these supplementary capacitors, extremelyhigh attenuation requirements can be met for power components in themicrowave range and extending all the way down through the UHF bands tothe VHF bands. .Still more remarkable is the fact that this wide-bandperformance is obtained by means of a relatively simple unitary circuitelement which provides uniformly distributed inductance and capacitance.If one chooses to use the supplementary capacitors shown in FIG. 4,satisfactory attenuation may be obtained at frequencies below onemegacycle.

In FIG. 3 is shown a filter rather similar to that of FIG. 1 which isequipped with certain fittings to facilitate connection of the filter inan electrical circuit. Employing like reference numbers for similarelements in FIGS. 1 and 3, it may be pointed out that magnetic material14 in FIG. 3 is retained in place by means of rubber disks 21 and 22,and that the ends of power conductor 11 are connected to feed-throughinsulators 23 and 24. Conductive tubing 12 is shown soldered to a metalsupporting disk 25 which is suitable for mounting in the wall of ascreened enclosure or shielded room in which sensitive tests are to bemade. The embodiment shown in FIG. 3 represents a working embodiment ofthe invention in which magnetic material 14 is formed into a body 5inches long and the assembly included within conductive tubing 12 whichis a cylinder 8 inches long. The power conductor 11 is helical inconfiguration and includes 35 turns of Wire, imparting to the filter aninductance of approximately 26 microhenries. The distributed capacitanceof the filter in this particular embodiment is approximately 3000micromicrofarads. Although ordinary powdered iron is used in themagnetic material 14 in the embodiments which have so far beendescribed, various experiments have been performed in order to determinewhether other magnetic materials might be sufficiently superior topowdered iron in order to warrant additional expense involved in the useof such materials. For instance, various ferrite materials have beentested in order to increase the inductance of the filter, and variousparticle sizes of magnetic material have been tried in order to optimizethe relationship between resistivity and permeability of the magneticmateral. Iron produced by the carbonyl process has been tested, andtests have even been made employing mixtures of iron and barium titanateas magnetic material 14 in order to maximize the dielectric constant ofthe material and hence the distributed capacitance of the filter.Although some minor improvements in performance have been made by meansof the use of these materials, such improvements have tended to takeplace in the higher-frequency ranges of the filter performance, wherethe performance was already entirely satisfactory when judged bycommonly applicable performance requirements. Moreover, the use of thesematerials other than iron has sometimes impaired the performance of thefilter in the low-frequency ranges where the performance requirementsare more critical. Hence, I presently favor the use of ordinary powderediron in the magnetic material in view of the fact that ordinary powderediron seems to give the best performance at the low frequencies and alsogives entirely satisfactory performance at the high frequencies. Onefurther elaboration which has been tried is the use of barium titanatein the form of a coating between the magnetic material and conductivetubing. Such use of barium titanate is predicated upon the assumptionthat the high dielectric constant of barium titanate will increase thedistributed capacitance of the filter sufficiently to improve itsperformance. Once again, it appears that the simpler configurationswithout the use of barium titanate give very satisfactory performance,and that adequate capacitance can be obtained without the use of thisdielectric material. The distributed capacitance can be enhanced byspecially shaping the inner surface of conductive tubing 12 as by thehelical threads shown A commercial configuration of filter according tomy invention is shown in FIG. 4 of the drawing, wherein supplementaryshunt capacitors 31 and 32 are employed at the respective ends of thefilter for improvement of the lower-frequency performance. The filter ofFIG. 4 'is designed to carry 30 amperes of power current throughconductor 11, and gives good attenuation performance at all frequenciesabove 1 megacycle per second. Even when these supplementary capacitorsare employed in order to improve the low-frequency performance of thefilter according to my invention, the capacitance of these supplementarycapacitors is but a small fraction of the capacitance required inprior-art filters.

One method by which a filter as shown in FIG. 4 may be assembledincludes the following steps:

(1) Insert the helical power conductor 11 through a hole in a rubberdisk 34.

(2) Connect the end of power conductor 11 to capacitor 31 by means of ascrew fitting 35.

(3) Insert conductor 11, rubber disk 34 and capacitor 31 into conductivetubing 12 and solder in place at one end of the conductive tubing.

(4) Insert some resin into conductive tubing 12 and bake the assembly inorder to adhere rubber disk 34 to the inner surface of conductive tubing12.

(5) Fill conductive tubing 12 up to a desired point with electricallyconductive magnetic material 14 such as the powdered iron mixturedesignated E-91-E by the McMillan Industrial Corporation, Ipswich, Mass.Again bake the assembly in order to stabilize mechanically the magneticmaterial 14. The E91-E mixture includes the following ingredients:

524.2 weight parts of Ancor 1025 iron powder as produced by HoeganaesSponge Iron Corp, Riverton, NJ.

51 weight parts of Epon 828 epoxy resin as produced by Shell ChemicalCompany.

43.8 weight parts of RD-l butyl glycidal ether as produced by Ciba Co.,Kimberton Pa. (This serves as a reactive diluent.)

3.8 weight parts of dimethylarninoethanol.

(This serves as a hardener when the mixture is baked.)

(6) Connect conductive link 37 to capacitor 32 by means of screw fitting38. Connect conductive link 37 at its other end to a connector block 39.

(7) While conductor 11 is stretched slightly so that its end extends outof the conductive tubing 12, fasten the end of filamentary conductor 11to connector block 39 and solder the connection.

(8) Insert capacitor 32 in place in the end of conductive tubing 12 andsolder at the end of conductive tubing 12.

In the embodiment as shown in FIG. 4, capacitors 31 and 32 are rated at600 volts DC, which is sufiicient insulation for most power supplieswhere a filter is needed. Power conductor 11 comprises a 4-inch coil ofNo. 12 copper wire coated with tin and formed into a helix having apitch of 7 turns per inch. The minimum clearance between helicalconductor 11 and the inner surface of conductive tubing 12 is less thaninch.

Before insertion of conductor 11 in conductive tubing 12, the innersurface of conductive tubing 12 is insulated with a film, approximately3 mils thick, of non-conductive material. In the filter according toFIG. 4, conductive tubing 12 has an outside diameter of 4 inch and is 7inches long. The wall thickness of the conductive tubing in one case is.022 where the tubing is of steel and in another case is .032 where thetubing is of brass.

The insulating coating which I favor and which is applied to the innersurface of conductive tubing 12 is a three-layer coating, each of whichis approximately 1 mil thick. I first apply a priming coat of polyvinylbutyral and then follow this priming coat by two successive coatscomprising phenolic resin and polyvinyl butyral. After applying each ofthese coats by dipping, spraying, or brushing, I prefer to bake the coatfor about 20 minutes at a temperature of approximately 250 F. Aninsulating coating as described gives adequate protection for potentialdifferences of at least 1000 volts at 400 cycles per second.

Obviously, there are many different insulating materials which may beemployed with satisfactory results. For instance, I have used a coatingloaded with aluminum powder between the priming coat and two subsequentinsulating coatings. For some frequencies it is possible to increase theattenuation by this means. I have also used an intermediate layercontaining barium titanate. However, for the sake of simplicity, Iprefer to use the three-layer coating as previously described.

It will be seen that a unitary aperiodic low-pass filter has beenprovided for accomplishing a filtering action which has previously beenperformed only by complicated arrangements of circuit components.However, the performance is far superior to the performance of the morecomplex prior art devices. The powdered iron or other magnetic materialserves not only as a magnetic core but also, by reason of its moderateconductivity, as a lossy dielectric. For best results, the conductivityof the magnetic material should be substantial but should beconsiderably less than that of the copper power conductor. In view ofthe high density of magnetic flux in the immediate vicinity of the powerconductor, it is desirable not to insulate this conductor, which wouldprevent contact with the magnetic material, but rather to insulate themagnetic material from the outer conductor at whose surface the magneticflux density is not so great.

It will be apparent that various modifications of my filter may be made,employing these principles without departing from the spirit and scopeof my invention as defined in the appended claims.

What is claimed is:

1. A low-pass filter comprising: an elongated electrically conductivesleeve member, said sleeve member when said filter is in operation,being grounded, a power conductor extending longitudinally of saidsleeve member, said conductor being spaced from and enclosed within saidsleeve member, a quantity of lossy magnetic material substantiallyfilling the space within said sleeve member, said lossy materialconsisting essentially of a mixture of finely divided electricallyconductive magnetic material and a resin, the electrical conductivity ofsaid mixture rendering said filter aperiodic in its performance, andinsulating means insulating said conductor electrically from said sleevemember, the thickness of said insulating means being of the order of notexceeding three one-thousandths of an inch.

2. A filter according to claim 1, wherein said lossy magnetic materialis in direct contact with said conductor.

3. A filter according to claim 1, wherein said insulating means is aninsulating sleeve disposed adjacent to and enclosed by said conductivesleeve member.

4. A filter according to claim 1, wherein said insulating meanscomprises at least one coating of electrically insulative materialapplied to the internal surface of said sleeve.

5. A filter according to claim 1, wherein said lossy material consistsessentially of:

524.2. weight parts of iron powder.

51 weight parts of epoxy resin.

43.8 weight parts of rbutyl glycidal ether. 3.8 weight parts ofdimethylaminoethanol.

6. A filter according to claim 1, wherein said magnetic material ispowdered iron.

7. A low-p=ass filter according to claim 1, further comprising at leastone shunt capacitor connected between said sleeve member and said powerconductor.

8. A low-pass filter comprising: an elongated tubular sleeve memberformed of electrically conductive material, a central power conductorextending longitudinally of said sleeve member coaxially therewith, saidconductor being spaced from and enclosed within said sleeve memher, aquantity of electrically conductive lossy material having effectivemagnetic permeability at frequencies higher than one megacycle, saidlossy material substantially filling the space within said sleeve membernot occupied by said conductor, said lossy material having aconductivity sufficiently high with respect to render said filtercontinuously aperiodic and increasingly attenuative throughout afrequency range extending upwardly from a predetermined minimumfrequency efness of the order of not exceeding three one-thousandths 4of an inch.

9. A low-pass filter according to claim 8, wherein said conductor is inthe form of a helix coaxial with said sleeve member.

10. A low-pass filter according to claim 8, wherein said lossy materialconsists essentially of a mixture of finely divided electricallyconductive magnetic material and a resin.

11. A low-pass filter according to claim 10, wherein said magneticmaterial is powdered iron and said resin is an epoxy resin.

12. A filter according to claim 1, wherein said insulating meanscomprises a layer of electrically insulative material interposeddirectly between said lossy material and said sleeve, said layer beingsufiiciently thin to permit said lossy material to suppress operation ofsaid sleeve as a coaxial transmission line.

13. A filter according to claim 8, wherein said insulating meanscomprises electrically insulative material in direct contact with saidsleeve member and with said lossy material, said lossy material andpower conductor forming an electrically conductive plug, said insulativematerial being sufficiently thin to suppress operation of said sleevemember and plug as a coaxial transmission line.

References Cited UNITED STATES PATENTS 2,238,915 4/ 1941 Peters 333-84,368,474 1/ 1945 Keister 333-79 2,409,640 10/ 1946 Moles 333-792,412,805 12/ 1946 Ford 333-79 2,759,155 8/1956 Hackenberg 333-792,782,381 2/1957 Dyke 333-79 2,838,735 6/ 1958 Davis 3-33-31 3,023,3832/ 1962 Sch-licke 333-79 3,035,237 5/ 1962 Schlicke 333-79 3,1 5,733 3/1964 Holinbeck 333-79 FOREIGN PATENTS 939,611 10/196-3 Great Britain.

818,775 8/1959 Great Britain. 1,205,158 10/1959 France. 1,205,158 11/1960 France.

HERMAN KARL SAALBACH, Primary Examiner.

C. BARAFF, Assistant Examiner.

1. A LOW-PASS FILTER COMPRISING: AN ELONGATED ELECTRICALLY CONDUCTIVESLEEVE MEMBER, SAID SLEEVE MEMBER WHEN SAID FILTER IS IN OPERATION,BEING GROUNDED, A POWER CONDUCTOR EXTENDING LONGITUDINALLY OF SAIDSLEEVE MEMBER, SAID CONDUCTOR BEING SPACED FROM AND ENCLOSED WITHIN SAIDSLEEVE MEMBER, A QUANTITY OF LOSSY MAGNETIC MATERIAL SUBSTANTIALLYFILLING THE SPACE WITHIN SAID SLEEVE MEMBER, SAID LOSSY MATERIALCONSISTING ESSENTIALLY OF A MIXTURE OF FINELY DIVIDED ELECTRICALLYCONDUCTIVE MAGNETIC MATERIAL AND A RESIN, THE ELECTRICAL CONDUCTIVITY OFSAID MIXTURE RENDERING SAID FILTER APERIODIC IN ITS PERFORMANCE, ANDINSULATING MEANS INSULATING SAID CONDUCTOR ELECTRICALLY FROM SAID SLEEVEMEMBER, THE THICKNESS OF SAID INSULATING MEANS BEING OF THE ORDER OF NOTEXCEEDING THREE ONE-THOUSANDTHS OF AN INCH.